As illustrated in FIG. 4, the solar cell module is composed of a plurality of solar cells and laminating the periphery thereof with a surface protection material (protection glass), an encapsulating material, and a back material (back sheet).
As a manufacturing process of a solar cell, for example, a texture structure is formed on the light receiving surface of a monocrystalline or polycrystalline silicon semiconductor substrate having a pn junction structure by using an alkaline solution, an antireflection film composed of silicon nitride is formed thereon by a chemical vapor deposition method, and thereafter, wiring is formed, thereby manufacturing a solar cell.
As a manufacturing process of a solar cell module, for example, a tab wire is soldered to a bus electrode on the surface of a solar cell to manufacture a string having a plurality of combined cells. Subsequently, both surfaces of the string are covered with an encapsulating material such as ethylene vinyl acetate (EVA), further, protection glass is laminated on the upper light receiving surface side and a back sheet is laminated on the back surface side. This laminated body is placed in a vacuum atmosphere at a reduced pressure and a high temperature and the encapsulating material is melted to paste and integrate the protection glass, the solar cell, and the back sheet, thereby manufacturing the solar cell module.
In addition, a transparent conductive film of indium tin oxide (ITO), zinc oxide (ZnO), or the like is formed on the light receiving surface of the semiconductor device, and the module is formed through the same manufacturing process as described above in some cases.
The solar cell module is fixed to a metal frame such as aluminum to be a panel. A plurality of panels are electrically connected, fixed to a frame, and installed outdoors. Systems that can generate electricity of the megawatt (MW) class by connecting a large number of panels have increased.
In this MW class power generation system, defects due to potential induced degradation (PID) frequently occur as a problem.
The condition under which the PID damage occurs is that this MW class power generation system is exposed to a high temperature and high humidity environment as well as the potential difference between both ends of the connected portion becomes several hundred volts or more since a large number of panels are connected.
The cause of the occurrence of PID damage is not yet clearly known. The PID damage is said to be caused because a large potential difference acts between the solar cell and the protection glass and sodium ions (Na+) contained in the protection glass diffuse to the outside from the protection glass due to the potential difference and accumulate on the surface of the antireflection film or the surface of the transparent electrode. For example, Non-Patent Document 1 proposes a model in which, as a result of the accumulation of Na+ ions on the surface of the antireflection film, electrons are attracted to the silicon substrate side of the antireflection film in order to maintain the neutral condition of charge, the surface of the silicon substrate located under the antireflection film is negatively charged, the n+ layer is locally inverted to the p+ layer, a leakage current is generated in the emitter layer, and the solar cell characteristics are degraded.
In order to suppress the PID damage, a method of improving the encapsulating material or a method of improving the protection glass has been conventionally proposed.
Examples in which the encapsulating material is improved are described below. Patent Document 1 proposes a method of suppressing the PID damage by increasing the thickness of the encapsulating material and designing the module so that the dielectric breakdown voltage is larger than the maximum system voltage. Patent Document 2 proposes a method in which a crosslinked cured film of a composition containing an ethylene-polar monomer copolymer and a crosslinking agent is used as the material for the encapsulating material, and the product of the volume resistivity and the thickness of the encapsulating material is set to be 5×1013 Ωcm2 or more to enhance the insulation property, thereby suppressing the diffusion of Na+ ions. Patent Document 3 proposes a method in which an ionomer resin layer having a small water vapor permeation amount and a high electric resistance even in a high humidity environment, a transparent resin layer having irregularities on both surfaces, and an ethylene-acetic acid copolymer resin layer are laminated between the protection glass and the encapsulating material in this order, thereby suppressing the diffusion of Na+ ions. Patent Document 4 proposes a method in which a crosslinking agent composed of an organic peroxide and a stabilizing agent composed of an oligomer are contained in addition to an encapsulating material composed of an ethylene vinyl acetate copolymer to improve the electrical insulation property and moisture shielding property, thereby suppressing the diffusion of Na+ ions.
Examples in which the protection glass is improved are described below. Non-Patent Document 2 proposes a method in which quartz glass which does not contain Na+ ions is used as the protection glass instead of soda-lime glass or a method in which silicon oxide is formed on the surface of soda-lime glass to suppress the diffusion of alkali metals. Non-Patent Document 3 states that the diffusion of Na+ ions can be suppressed when a titanium oxide (TiO2) film is formed on the surface of a protection glass by a wet method and the film thickness is 100 nm or more. However, the effect of suppressing PID is incomplete as degradation of the solar cell characteristics appears after a PID test has been conducted for 2 hours. Non-Patent Document 4 reports that the film framework exhibits a columnar crystal framework although a TiO2 film having a film thickness of 1 μm is formed on the surface of the protection glass by the sputtering method, Na+ ions diffuse through the crystal grain boundaries, and it is thus impossible to suppress the PID damage. Non-Patent Document 5 reports that diffusion of Na+ ions can be delayed by further layering a SiO2 film in addition to a TiO2 film. Patent Document 5 proposes a method in which the Na+ ion concentration contained in the protection glass surface is set to 0.01 wt % or more and 13 wt % or less in terms of Na2O and the volume resistivity is set to 1.0×108.3 Ωcm or more. Patent Document 6 proposes a method in which the surfaces of the protection glass and the frame are coated with a hydrophobic film, thereby suppressing the dissolution of Na+ ions.
As another method, a method in which a transparent conductive film is formed on the antireflection film, the transparent conductive film and the back surface of the solar cell are electrically short-circuited and grounded, thereby preventing the accumulation of electric charges in the vicinity of the surface of the antireflection film, and the like are proposed as in Patent Document 7.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2014-11270
Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2014-27034
Patent Document 3: Japanese Unexamined Patent Application, Publication No. 2014-157874
Patent Document 4: Japanese Unexamined Patent Application, Publication No. 2014-212318
Patent Document 5: PCT International Publication No. WO2014/057890
Patent Document 6: U.S. Published Patent Application Publication, No. 2014/0150850, Specification
Patent Document 7: U.S. Pat. No. 7,786,375, Specification
Non-Patent Document 1: J. Bauer et al., Physica Status Solidi RRL, Vol. 6, pp. 331-333(2012)
Non-Patent Document 2: P. Hacke et al. Proceeding 25th EUPVSEC, pp. 3760-3765(2010)
Non-Patent Document 3: K. Hara et al., The Royal Society of Chemistry Advances, Vol. 4, pp. 44291-44295(2014)
Non-Patent Document 4: E. Aubry et al., Surface & Coatings Technology, Vol. 206, pp. 4999-5005(2012)
Non-Patent Document 5: J. Zita et al., Journal of Photochemistry and Photobiology A: Chemistry, Vol. 216, pp. 194-200(2010)